[Doc] Add DGLSubGraph, and DGLGraph.readonly() interface to docs (#421)

* fix rgcn tutorial

* small fix

* upd

* findedge/s

* upd

* upd

* upd

* upd

* add test

* remove redundancy

* upd

* upd

* upd

* upd

* add edge_subgraph

* explicit cast

* add test immutable subg

* reformat

* reformat

* fix bug

* upd

* hotfix

* subgraph docs

docs/source/api/python/graph.rst

@@ -56,6 +56,7 @@ Transforming graph
     DGLGraph.edge_subgraph
     DGLGraph.line_graph
     DGLGraph.reverse
+    DGLGraph.readonly
 
 Converting from/to other format
 -------------------------------

docs/source/api/python/subgraph.rst

+.. _apigraph:
+
+DGLSubGraph -- Class for subgraph data structure
+=========================================
+
+.. currentmodule:: dgl
+.. autoclass:: DGLSubGraph
+
+ Mapping between subgraph and parent graph
+ -------------------------------------
+    :toctree: ../../generated/
+
+    DGLSubGraph.parent_nid
+    DGLSubGraph.parent_eid
+    DGLSubGraph.map_to_subgraph_nid  
+
+Synchronize features between subgraph and parent graph
+------------------------------------------------------
+    :toctree: ../../generated/
+
+    DGLSubGraph.copy_from_parent
+    DGLSubGraph.copy_to_parent